In a statement released late Monday, police said they had received a letter from the Public Security Department in neighboring Guangdong province stating that they "understood that Lee Po is in the mainland," using a different English spelling of his name.

Also enclosed was a letter from Lee to the Hong Kong government, which the statement said was similar to one received by his wife January 17 that was widely reported on in local media.

Lee went missing on December 30 and was last seen near his company's warehouse. He holds a British passport and a spokesperson for the British Foreign and Commonwealth Office said it remained "deeply concerned" about the possible detention of Lee and his colleagues.

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Three other associates of the publisher Mighty Current have gone missing. It specializes in books often critical of China's ruling elite and also owns a bookstore, Causeway Bay Books.

According to Hong Kong pro-democracy lawmaker Albert Ho, the publishing company was said to have been working on a book detailing Chinese President Xi Jinping's love affairs.

Their disappearance has sparked angry protests in Hong Kong, where pro-democracy politicians believe Lee was seized in the city by Chinese security forces.

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Rights activists said that Gui's reappearance reinforced concerns about the fate of his associates and have voiced skepticism over the nature of his televised confession.

"We can't exclude that Gui Minhai's videotaped 'confession' wasn't made under duress. We want to ensure he has access to a lawyer, as mandated under Chinese law," said Nicholas Bequelin, East Asia Director at Amnesty International.

Gui Minhai's sudden desire to return to China was said to have been driven by the news that his father had passed away and a wish to spend time with his elderly mother.

"Going back to my country and turning myself in was voluntary. This was not related to others," he told Chinese state television CCTV in an interview.